The purpose of the research in this proposal is to determine how signal processing in central oculomotor pathways, particularly those related to the vestibulo-ocular reflex (VOR), are modified by active head movements and target distance. The studies proposed will be focused primarily on 1) recording the response properties of identified secondary vestibulo-ocular reflex neurons and neurons in the vestibulo-cerebellum in alert, head free squirrel monkeys and 2) recording from the same cells during combined angular and linear vestibular stimuli and during vergence related gain changes in the VOR. There are three specific aims: 1. The firing behavior of different classes of identified secondary vestibulo-ocular reflex neurons related to the horizontal semicircular canal or to horizontal eye movements will be recorded in head-free monkeys during a variety of different types of head movements, particularly pursuit of moving visual targets. The contribution of neck afferent inputs to the response of the cells during head pursuit will be assessed by passive rotation of the body while fixing the position of the head in space, and by comparing the responses evoked by passive head and neck rotation to passive whole body rotation. The goal of these studies is to determine what signals horizontal vestibulo-ocular reflex neurons generate during active head movements. 2. The firing behavior of secondary vestibulo-ocular reflex neurons during different combinations of angular and linear head acceleration will be studied. The changes in cellular responses associated with near target viewing will be evaluated, and the contribution of different classes of secondary VOR neurons to the parametric gain changes in the VOR will be determined. 3. Single unit recordings will be made from Purkinje cells in the vestibulo-cerebellum using the paradigms described in the first and second aims. Their responses will be studied during different combinations of angular and linear head acceleration, and during parametric gain changes in the AVOR and LVOR related to viewing distance. The results of these studies are expected to increase our knowledge of the neural mechanisms that are utilized to maintain stable, binocular vision of objects at different distances during active and passive head movements.